The field of invention pertains to a system that combines an IC engine with a fuel processor to achieve a system that consumes hydrocarbon fuels and generates and stores hydrogen with high efficiency and low operation cost.
Hydrogen as a fuel has attracted increasing attention. The advantages of hydrogen fuel include: a) fuel cells, using hydrogen as fuel, can achieve thermal efficiency higher than 60% (thermal efficiency=electric energy output/thermal energy input); b) hydrogen fuel is considered zero-emission fuel since the consumption of hydrogen only yields water. However, storage and distribution of hydrogen on a large scale is capital and energy intensive, which hinders the widespread use of hydrogen fuel in the economy. Currently, the majority of hydrogen production is via the route of natural gas steam reforming in large scale hydrogen plants. After many years of optimization, this process has achieved hydrogen thermal efficiency of 84% or higher (hydrogen efficiency=lower heating value of hydrogen output/lower heating value of natural gas input). Heating value is the amount of energy released when a fuel is completed combusted in a steady-flow process and the products are returned to the state of reactants. When product water is in vapor form, the heating value is called lower heating value (LHV). LHV is a direct indication of the energy release when a certain fuel is completely combusted. Hydrogen has one of the highest heating value among fuels, for instance, LHVH2=120 kJ/gram, LHVCH4=50 kJ/gram, LHVgasoline=43 kJ/gram. However, due to the low molecular weight of hydrogen, energy per volume of hydrogen at room temperature and atmospheric pressure is low, for instance, LHVH2=10.2 kJ/liter, LHVCH4=33.8 kJ/liter, LHVgasoline=31.8×103 kJ/liter. Therefore, the cost for distribution and storage per unit of energy of hydrogen is significantly higher than that of natural gas and even more so in comparison to that of gasoline. As a result, the economics as well as the energy efficiency for long distance distribution of hydrogen are not favorable.
An alternative to centralized hydrogen plants with a distribution network is on-site hydrogen generation. Hydrogen may be generated on demand using small-scale reformer systems (e.g. several hundred kilograms per day) with minimal requirements for hydrogen storage. The US Department of Energy (USDOE) has set a cost target for on-site hydrogen production of $1.50 energy cost per kilogram of hydrogen produced and stored at 2,300 psi, which is equivalent to $12.50/million kJ or $11.80/million Btu. A low-pressure spherical storage tank may have an operation pressure in the range of 1,700-2,300 psi. On the other hand, the maximum operation pressure for a high pressure storage vessel can reach 4,500 psi or above. The energy costs of natural gas and electricity in the recent years are about $4.4-$6.0/million Btu and $20.51/million Btu (i.e. $0.07/kWhr), respectively. At this electricity rate, it is estimated the electricity cost to compress hydrogen from atmospheric pressure to a storage pressure of 2,300 psi or above is more than $3.00 /million Btu. This exceeds the target cost for energy consumption to produce hydrogen. Clearly the electricity consumption in the system needs to be minimized. If the only energy input to the system is in form of natural gas (i.e. no electricity) the system efficiency needs to exceed 42.3%-58.3%, varying according to natural gas market price, to meet the DOE hydrogen cost target.